Heretofore, in order to keep the humidity within a controlled chamber constant, through the use of an ultrasonic humidifier, the humidity within the controlled chamber has been controlledby such ON-OFF-operations as are shown in FIG. 7.
That is to say, as shown in FIG. 7, an ON-humidity H.sub.ON and OFF-humidity H.sub.OFF having a hysteresis .DELTA.H for an objective humidity H.sub.O are determined, the humidity within the controlled chamber is detected with a humidity detector, at the time points t.sub.1 and t.sub.3 when the detected humidity reaches the OFF-humidity H.sub.OFF while rising, the oscillation of the oscillator of the ultrasonic humidifier is fuller stopped and, at the time points t.sub.2 and t.sub.4 when the detected humidity H reaches the ON-humidity while lowering, the oscillator of the ultrasonic humidifier is operated with an amplitude of 100%.
In the case shown in FIG. 7, the detected humidity H at the control starting time point t.sub.0 is so low that the initiated operation of the oscillator of the ultrasonic humidifier must be with an amplitude of 100%.
However, as the point at which the ultrasonic humidifier is placed and the point at which the humidity detector is placed are usually spaced from each other, the time of transmitting air humidified by spray is comparatively long and the time it takes for the humidity detector to sense the level of humidity itself can not be neglected. Therefore, in the known humidification controlling system, the transmission delay and wasted time are generally so large that, in the above mentioned conventional humidification controlling system, there have been defects that, as shown in FIG. 7, an overshoot .DELTA.A and undershoot .DELTA.B is disadvantageously generated. Therefore a cycling (continued oscillation) of a large amplitude will occur and the humidity within the controlled chamber will not be able to be precisely kept constant.
Also, in the conventional humidification controlling system, as described above, the hysteresis .DELTA.H is provided to somewhat reduce the frequency of the ON-OFF-operations (the oscillation of 100% amplitude and the precise stop of the oscillation) of the oscillator of the ultrasonic humidifier. However, the precision will be further reduced by the part of the hysteresis .DELTA.H, therefore the hysteresis can not be made so large and therefore the above mentioned oscillator will be so high in the ON-OFF frequency as to be likely to be broken.